This invention relates to a transistor circuit for obtaining a large amplitude voltage signal from a transistor circuit such as the collector of the transistor.
A large amplitude A-class amplifying circuit is used in an operational amplifying circuit or in a low frequency power amplifying circuit or the like as a circuit for driving a push-pull output stage. In this circuit, driving transistors are caused to operate in the saturation region at the time of maximum voltage output or minimum voltage output.
The inventor has found that when the amplifying transistors operate in the saturation region or in the quasi-saturation region in this manner, the following problem occurs.
When an amplifying transistor operates in the saturation region (i.e., the operation region in which the collector-emitter voltage V.sub.CE is about 0.25-0.4 V), the output voltage-vs.-distortion ratio characteristic becomes a so-called "hard-clip" characteristic and comes to contain higher harmonics of especially high values. Hence, the output distortion increases and has an adverse effect on the tone quality in an audio amplifier.
On the other hand, the amplifying transistor operates in the quasi-saturation region (i.e., the operation region in which the collector-emitter voltage is about 1 V) before it reaches the saturation region. In the operation of the transistor in this quasi-saturation region, the cut-off frequency f.sub.T markedly lowers as shown in FIG. 5 whereby the phase shift of the transistor becomes great and, consequently, causes an increase in the instability of the nagative feed-back action in the negative feed-back circuit.
When, for example, a driving transistor in a large amplitude A-class transistor amplifying circuit for driving two push-pull output transistors acting between two power source voltage +V and -V, is driven into its quasi-saturation region, the push-pull output V.sub.OUT drops to a point immediately above the power source voltage -V. In this instance, the push-pull output V.sub.OUT is caused to exhibit a local oscillation wave form V.sub.OCS such as shown in FIG. 4 due to a large phase shift in the negative feed-back circuit resulting from a remarkable lowering in the cut-off frequency f.sub.T.